park on an automating transmission engages a parking pawl in the tranny to hold the mechanism in place its a large gearlike piece and the pawl is a lever that comes down and finds a place to hold on the big gear part thats why sometimes it rolls a few inches and then grabs and why it can be hard to shift from park on a hill the weight of the vehicle is on the parking pawl a emergency brakeparking breakhand brake is usually a cable operated mechanism that engages the rear brakes it can be a hand operated one or a pedal one and some parking brakes are electric these days